Electron Attachment Processes Research Articles

Dissociative properties of C4F6 obtained using computational chemistry

The electronic properties of C4F6 were investigated by using computational chemistry to clarify the dissociative channels in the process plasma. The results show the mainly ionized ion is C3F3 + (CF2=C=CF+; propargyl ion) which is observed in the mass spectrum with the electron energy of 70 eV, and the intermediate molecular structure to produce C3F3 + ion is methyl allene ion (CF2=C=CF–CF3 +). The molecular ion C4F6 + is also mainly produced in the ionization threshold region. For the excited states, the calculated results suggest that CF2CFCF + CF2 dissociation takes place in the energy region higher than 7.0 eV and C2F3 + C2F3 dissociation takes place in the energy region higher than 8.0 eV. In the electron attachment process, the vertical electron attachment energy was calculated as 1.1 eV and the nonadiabatic negative ion energy was −0.2 eV lower than the energy of neutral C4F6.

Observation of Renner-Teller and predissociation coupled vibronic intensity borrowing in dissociative electron attachment to OCS.

We use a time-of-flight-based velocity map imaging method to look into the dissociative electron attachment to a linear OCS molecule at electron beam energies ranging from 4.5 to 8.5eV. The conical time-gated wedge slice imaging method is utilized to extract fragments' slice images, kinetic energy (KE), and angular distributions, which provide a complete kinematic understanding of this experiment on the dissociative electron attachment process. We observe that the formation of S- is relatively higher than the O- product. Three distinct dissociative KE bands of S-/OCS have been observed for the 5.0 and 6.5eV resonance positions. We notice a prominent rovibrationally coupled bimodality for each KE band in the variation of the most probable KE values. When the electron energy is changed from 5.5 to 6.0eV, we observed vibronic intensity borrowing in the highest momentum band of S- via the Σ → Π symmetric dipole-forbidden transitions within the 1.5eV energy gap. Multiple peaks in the angular distributions of S- and their modeling indicate the presence of Renner-Teller vibronic splitting. Using Q-Chem's implemented complex absorbing potential-equation of motion-electron affinity coupled cluster singles and doubles aug-cc-pVDZ+4s3p level of multireference-based electronic structure theory, we confirm the presence of OCS temporary negative ion bending vibrations and Renner-Teller vibronic splittings for the Π symmetric states. Additionally, we notice the presence of a non-radiative predissociation continuum (bringing down the rotational spectrum) and speed-dependent angular anisotropy in the S- fragmentation. Our findings at the resonance of OCS at 6.5eV closely align with the prediction of vibronic intensity borrowing by Orlandi and Siebrand [J. Chem. Phys. 58, 4513 (1973)].

Theoretical Investigation on excited dipole bound states of alkali-containing diatomic anions

Abstract Information about electronic excited states of molecular anions plays an important role in investigating the electron attachment and detachment processes. Here we present a high-level theoretical study on the electronic structures of twelve alkali-metal-containing diatomic anions MX- (MX = LiH, LiF, LiCl, NaF, NaCl, NaBr, RbCl, KCl, KBr, RbI, KI, and CsI). The equation-of-motion electron-attachment coupled-cluster singles and doubles (EOM-EA-CCSD) method is carried out to calculate the electron binding energies (EBEs) of 10 electronic excited states of each of the twelve molecule anions. With addition of different s-/p-/d- diffusion functions in the basis set, we have identified possible excited dipole bound states (DBSs) of each anion. With the investigation of EBEs on the twelve MXs with dipole moment (DM) up to 12.1 D, we evaluate the dependence of the number of anionic excited DBSs on molecular DM. The results indicate that, there exists at least two or three DBSs of anions with the molecular DM larger than 7 D and a molecule with DM > 10 D can sustain a π-DBS of the anion. Our study would shed some implications on the excited DBSs electronic states of alkali-metal-containing diatomic molecules.

New emission band of solid nitrogen

New results on the study of radiation effects in solid nitrogen and N2-doped Ne matrix are presented, with a focus on the so-called γ-line origin. The irradiation was carried out in dc regime with an electron beam of subthreshold energy. The relaxation dynamics was monitored by emission spectroscopy: cathodoluminescence (CL) and nonstationary luminescence (NsL), along with current activation spectroscopy. Thermally stimulated luminescence (TSL) and exoelectron emission (TSEE) of pure nitrogen and N2 in the Ne matrix were measured in a correlated manner. Three emission bands were recorded in the NIR CL spectra of solid N2: 794, 802, and 810 nm. The band at 810 nm was detected for the first time. These three bands are characterized by similar behavior and form molecular series with spacing between adjacent vibrational energy levels of the ground state of 125 and 123 cm−1. These data cast doubt on the recently made assumption that the γ-line is attributed to the emission of the nitrogen anion N− [R. E. Boltnev, I. B. Bykhalo, I. N. Krushinskaya et al. Phys. Chem. Chem. Phys. 18, 16013 (2016)]. The processes of electron attachment and neutralization of positively charged species are discussed. It has been established that the γ-line in the TSL spectra of pure nitrogen and N2-doped Ne matrix correlates with TSEE currents and recombination emission of O+, N2+, and N4+ ions, which indicates its connection with the neutralization reaction. The measurement of NsL supported this conclusion. A new possible assignment of the γ-line and its satellites to the emission of tetranitrogen N4 is discussed.

<i>Ab initio</i> molecular dynamics study on dissociation process of 2-thiouracil and its tautomers under low-energy electron interactions

When biomolecules interact with high-energy particles and rays, they are directly ionized or dissociated, then a large number of low-energy electrons are formed as secondary particles. These low-energy electrons will attach to biomolecules, and trigger off the secondary dissociation, forming free radicals and ions with high reactivity, which can damage the structure and function of the biomolecule and cause irreversible radiation damage to the biomolecule. It is important to study the low-energy dissociative electron attachment (DEA) process of biomolecules for understanding radiation damage to biological organisms. Currently, the theoretical studies of DEA have mainly focused on the bound states of negative ions and the types of resonances in the dissociation process. The dissociation process is well described by quantum computational method, but the diversity and complexity of dissociation channels present in the dissociation process of 2-thiouracil molecule also pose a great computational challenge to these methods. In addition, the quantum computational methods are not ideal for dealing with the discrete states of chemical bonds and the problem of continuity coupling of electrons. The dissociation dynamics of biomolecules mainly results from ionization and electron attachment. <i>Ab initio</i> molecular dynamics simulation can reasonably describe these processes. In light of these considerations, <i>ab initio</i> molecular dynamics simulation is used in this work to study dynamic variation process in DEA. The low-energy electron dissociative attachment to 2-thiouracil in the gas phase is studied by using the Born-Oppenheimer molecular dynamics model combined with density functional theory. It is found that an important dehydrogenation phenomenon of 2-thiouracil and its tautomers occurs in the DEA process, and that the N—H and C—H bond are broken at specific locations. Due to the loss of hydrogen atoms at the N and C sites, the closed-shell dehydrogenated negative ion (TU-H)<sup>–</sup> forms, which is the most important negative ion fragments in the dissociation process. The potential energy curves, the bond dissociation energy and the electron affinity energy of the broken bond show that the N—H bond is the most likely to break, indicating the formation of the negative ion (TU-H)<sup>–</sup> mainly comes from the breaking of N—H bond. The theoretical calculations in this work are in good agreement with the available experimental results, indicating that the chosen calculation method is fully reliable. ​The BOMD simulations can not only dynamically recover the process of dissociative attachment of low-energy electrons to 2-thiouracil, but also more importantly provide an insight into the mechanisms of dehydrogenation and dissociation channels of 2-thiouracil molecules in DEA process.

New approach to instantaneous polarizable electrostatic embedding of the solvent

The sequential quantum mechanics/molecular mechanics (S-QM/MM) method efficiently computes solvent effects on the electronic properties of solutes. The protocol involves two steps: solute-solvent configurations are generated from MM simulations, while the solute properties are computed in the subsequent QM step. A well-known difficulty within the S-QM/MM framework is the description of the polarization of the MM partition (electrostatic embedding) in case the electronic structure of the solute undergoes sudden changes, such as in electronic excitation, electron attachment or detachment processes. To improve the description of the electronic polarization of the solvent, we propose the self-consistent S-QM/MM polarizable electrostatic embedding (scPEE-S-QM/MM) method, which generates individual atomic charges for the solvent molecules due to solute-solvent and solvent-solvent polarization. The electronic properties calculated for the neutral and anion states of 1-methyl-4-nitroimidazole were found to be significantly affected by solvent polarized electrostatic embeddings obtained for different solute electronic states. Finally, the method also provides accurate results for bulk water.

The performance of approximate equation of motion coupled cluster for valence and core states of heavy element systems

The equation of motion coupled cluster singles and doubles model (EOM-CCSD) is an accurate, black-box correlated electronic structure approach to investigate electronically excited states and electron attachment or detachment processes. It has also served as a basis for developing less computationally expensive approximate models such as partitioned EOM-CCSD (P-EOM-CCSD), the second-order many-body perturbation theory EOM (EOM-MBPT(2)) and their combination (P-EOM-MBPT(2)) [S. Gwaltney et al., Chem. Phys. Lett. 248, 189–198 (1996)]. In this work we outline an implementation of these approximations for four-component based Hamiltonians and investigate their accuracy relative to EOM-CCSD for valence excitations, valence and core ionisations and electron attachment, and this for a number of systems of atmospheric or astrophysical interest containing elements across the periodic table. We have found that across the different systems and electronic states of different nature considered, partition EOM-CCSD yields results with the largest deviations from the reference, whereas second-order based approaches tend show a generally better agreement with EOM-CCSD. We trace this behaviour to the imbalance brought about by the removal of excited state relaxation in the partition approaches, with respect to degree of electron correlation recovered.

Algebraic Diagrammatic Construction Schemes Employing the Intermediate State Formalism: Theory, Capabilities, and Interpretation.

Algebraic diagrammatic construction (ADC) schemes represent a family of ab initio methods for the calculation of excited electronic states and electron-detached and -attached states. All ADC methods have been demonstrated to possess great potential for molecular applications, e.g., for the calculation of absorption or photoelectron spectra or electron attachment processes. ADC originates from Green's function or propagator theory; however, most recent ADC developments heavily rely on the intermediate state representation or effective Liouvillian formalisms, which comprise new ADC methods and computational schemes for high-order properties. The different approaches for the calculation of excitation energies, ionization potentials, and electron affinities are intimately related, and they provide a coherent description of these quantities at equivalent levels of theory and with comparable errors. Most quantum chemical program packages contain ADC methods; however, the most complete ADC suite of methods can be found in the recent release of Q-Chem.

Water-assisted electron capture exceeds photorecombination in biological conditions.

A decade ago, an electron-attachment process called interatomic Coulombic electron capture has been predicted to be possible through energy transfer to a nearby neighbor. It has been estimated to be competitive with environment-independent photorecombination, but its general relevance has yet to be established. Here, we evaluate the capability of alkali and alkaline earth metal cations to capture a free electron by assistance from a nearby water molecule. We introduce a characteristic distance rIC for this energy transfer mechanism in equivalence to the Förster radius. Our results show that water-assisted electron capture dominates over photorecombination beyond the second hydration shell of each cation for electron energies above a threshold. The assisted capture reaches distances equivalent to a fifth to seventh solvation shell for the studied cations. The far reach of the assisted electron capture is of significant general interest to the broad spectrum of research fields dealing with low-energy electrons, in particular radiation-induced damage of biomolecules. The here introduced distance measure will enable quantification of the role of the environment for assisted electron attachment.

Dissociation channels of c-C4F8 to C2F4 in reactive plasma

Progress in computational methods and personal computing has made possible more accurate estimations for primary dissociation channels and energies. The main dissociation route is revealed to be via the 7E excited state with an energy of 12.23 eV, which is composed of transitions from the highest occupied molecular orbital with b1 symmetry to some degenerate unoccupied e molecular orbitals. The main contributing e orbitals consisted of antibonding combination of C2F4 π-bonding orbitals. This degenerate 7E state is lowered by non-adiabatic transitions through the conical interactions on the dissociating route to 2C2F4, so the energy is finally relaxed at the dissociative second lowest 1E excited state leading to 2C2F4 production. In the electron attachment process, the calculated results show that the F− ion is produced from the excited states of the D4h c-C4F8 − ion through conical interactions at the energies of 4.3 eV, 5.6 eV, and 5.0 eV, along the C–F dissociation route.

Electron and Positron Scattering Cross Sections from CO2: A Comparative Study over a Broad Energy Range (0.1–5000 eV)

In this Review, we present a comparative study between electron and positron scattering cross sections from CO2 molecules over a broad impact energy range (0.1–5000 eV). For electron scattering, new total electron scattering cross sections (e-TCS) have been measured with a high resolution magnetically confined electron beam transmission system from 1 to 200 eV. Dissociative electron attachment processes for electron energies from 3 to 52 eV have been analyzed by measuring the relative O– anion production yield. In addition, elastic, inelastic, and total scattering cross section calculations have been carried out in the framework of the Independent Atom Model by using the Screening Corrected Additive Rule, including interference effects (IAM-SCARI). Based on the previous cross section compilation from Itikawa (J. Phys. Chem. Ref. Data, 2002, 31, 749−767) and the present measurements and calculations, an updated recommended e-TCS data set has been used as reference values to obtain a self-consistent integral cross section data set for the elastic and inelastic (vibrational excitation, electronic excitation, and ionization) scattering channels. A similar calculation has been carried out for positrons, which shows important differences between the electron scattering behavior: e.g., more relevance of the target polarization at the lower energies, more efficient excitation of the target at intermediate energies, but a lower total scattering cross section for increasing energies, even at 5000 eV. This result does not agree with the charge independence of the scattering cross section predicted by the first Born approximation (FBA). However, we have shown that the inelastic channels follow the FBA’s predictions for energies above 500 eV while the elastic part, due to the different signs of the scattering potential constituent terms, remains lower for positrons even at the maximum impact energy considered here (5000 eV). As in the case of electrons, a self-consistent set of integral positron scattering cross sections, including elastic and inelastic (vibrational excitation, electronic excitation, positronium formation, and ionization) channels is provided. Again, to derive these data, positron scattering total cross sections based on a previous compilation from Brunger et al. (J. Phys. Chem. Ref. Data, 2017, 46, 023102) and the present calculation have been used as reference values. Data for the main inelastic channels, i.e. direct ionization and positronium formation, derived with this procedure, show excellent agreement with the experimental results available in the literature. Inconsistencies found between different model potential calculations, both for the elastic and inelastic collision processes, suggest that new calculations using more sophisticated methods are required.

Structural, spectroscopic and electron collisional studies of isoxazole (C3H3NO)

• Comprehensive structural, spectroscopic and electron collision data of Isoxazole. • DFT calculations of ground state geometry and vibrational frequencies. • Comparison with earlier data; confirmation/revision of vibronic assignments. • Vertical excitation energies for singlet and triplet states using TDDFT. • Low energy electron scattering data for Isoxazole from 0.1-20 eV. The structural, spectroscopic, and electron collision data of Isoxazole (C 3 H 3 NO) were subjected to extensive computational studies. Spectroscopic calculations were performed on ground and excited states using the density functional theory (DFT) and the time dependent density functional theory (TDDFT) respectively. Ground state geometry and vibrational frequencies agree well with earlier works and lead to re-assignment of a few vibrational bands. Theoretically predicted singlet and triplet excitation energies are in good agreement with earlier experimental data, while predicted oscillator strengths of singlet states correlate well with the observed relative spectral intensities. The electron collision data for isoxazole molecule were calculated using Quantemol-N, which employs the UK Molecular R-matrix codes. These calculations were performed for the total elastic cross-sections along with their symmetrical components, differential and momentum transfer cross-sections, inelastic scattering cross-sections comprising of discrete electronic excitation, ionization and dissociative electron attachment processes. We employed the two most popular theoretical approaches to calculate the electron impact ionization cross-sections viz. Complex scattering potential-ionization contribution (CSP-ic) and the Binary encounter Bethe (BEB) model. The rate coefficients for elastic and inelastic processes were estimated over a wide electron temperature range. In addition to scattering calculations, the other motivation was to obtain resonances which are signatures of many significant low energy phenomena. The first two peak correspond to π* shape resonances which are observed at 2.1 eV and 4.69 eV and are in accordance with the earlier reported data. The third peak is observed at 10.96 eV which may be attributed to the σ* shape resonance.

A Missing Puzzle in Dissociative Electron Attachment to Biomolecules: The Detection of Radicals

Ionizing radiation releases a flood of low-energy electrons that often causes the fragmentation of the molecular species it encounters. Special attention has been paid to the electrons’ contribution to DNA damage via the dissociative electron attachment (DEA) process. Although numerous research groups worldwide have probed these processes in the past, and many significant achievements have been made, some technical challenges have hindered researchers from obtaining a complete picture of DEA. Therefore, this research perspective calls urgently for the implementation of advanced techniques to identify non-charged radicals that form from such a decomposition of gas-phase molecules. Having well-described DEA products offers a promise to benefit society by straddling the boundary between physics, chemistry, and biology, and it brings the tools of atomic and molecular physics to bear on relevant issues of radiation research and medicine.

Formation of negative-ion resonance and dissociative attachment in collisions of NO2 with electrons

The process of electron attachment to the NO2 molecule is investigated theoretically using an approach based on a study by O’Malley (1966 Phys. Rev. 150 14). The approach combines the normal mode approximation for representation of vibrational dynamics of NO2 and one-dimensional treatment, along each normal mode, of the attachment process as in O’Malley’s theory, such that only a modest computational effort is required to compute the attachment cross section. Taking into account the survival probability of the formed resonant state of , the cross section for dissociative electron attachment to NO2 is also estimated. To compare with available experimental data, the theoretical cross section is convoluted with energy distribution of NO2–e− collisions with uncertainties reported in experimental studies. Peak values of the convoluted theoretical cross section are found to be about a factor of 2–10 larger than the experimental results.

CAP/EA-ADC method for metastable anions: Computational aspects and application to π* resonances of norbornadiene and 1,4-cyclohexadiene.

The second- and third-order algebraic-diagrammatic construction schemes for the electron propagator for studies of electron attachment processes [EA-ADC(2) and EA-ADC(3)] have been extended to include the complex absorbing potential (CAP) method for the treatment of electronic resonances. Theoretical and conceptual aspects of the new CAP/EA-ADC methodology are studied in detail at the example of the well-known 2Πg resonance of the nitrogen anion N2 -. The methodology is further applied to π* shape resonances, for which ethylene is considered as a prototype. Furthermore, the first many-body treatment of the π+ * and π- * resonances of norbornadiene and 1,4-cyclohexadiene is provided, which have served as model systems for the concept of through-space and through-bond interactions for a long time.

Vibrational excitation and dissociation of deuterium molecule by electron impact

A theoretical investigation of electron-D2 resonant collisions—via the low-lying and the Rydberg states of D—is presented for vibrational excitation, dissociative electron attachment and dissociative excitation processes by using the local-complex-potential approach. Full sets of vibrationally resolved cross sections, involving the ground electronic state——and the first two electronic excited states— and —of the D2 molecule, are given for fusion plasma applications in their technologically relevant partially dissociated, detached divertor regimes. In particular, transitions between electronic excited states are also considered. Comparisons are made with cross sections present in the literature, where available.

(Invited) Plasma Electrochemistry: Plasmas Are so Much More Fun for Electrochemistry Than Liquids!

As electrochemists, we are interested in electron attachment and detachment processes. Traditionally, we control the availability of electrons via an electrically conducting solid and measure electron transfer across the solid/liquid interface. Of course, there are exceptions to this picture, e.g. liquid/liquid interfaces, but often liquids are involved to provide an electrolyte medium to support the chemical species. Gaseous electrolytes have typically been ignored due to their feeble electrical conductivity. However, recently with the advent of new accessible approaches to form stable plasmas, these electrically conducting gases are attracting some significant interest and are now being investigated as exotic electrochemical environments.The defining property of plasmas is presence of free electrons; because of this they may be considered as both electrodes or electrolytes1, 2. We describe results supporting both modes.As electrodes, we show that metal oxides on surfaces may be reduced to zero valent metals using a helium atmospheric plasma jet.3 We show that free electrons do indeed reduce a copper oxide film, which may be carefully controlled by surface bias.4 A gaseous flame doped with electroactive species may be considered as electrolytes. Using a three-electrode system5, we may measure unique voltammograms for a series of small organic molecules and amino acids.6 Except for leucine and isoleucine, all were distinguishable. The reduction signatures originate from specific electron attachment reactions of radicals formed via incomplete combustion and fragmentation of the parent molecules. In this case without a solvent we have an extended potential window and our voltammograms extend between 0 and -10 V, which gives unprecedented access to chemistry not previously accessible in liquids. Moreover, mass transport properties are far better than in liquids, as such the fluxes of electroactive species to the electrode a much greater. References Rumbach, P., Bartels, D.M., Sankaran, R.M. & Go, D.B. The solvation of electrons by an atmospheric-pressure plasma (vol 6, pg 7248, 2015). Nature Communications 7 (2016).Elahi, A., Fowowe, T. & Caruana, D.J. Dynamic Electrochemistry in Flame Plasma Electrolyte. Angewandte Chemie-International Edition 51, 6350-6355 (2012).Sener, M.E., Palgrave, R., Quesada Cabrera, R., Caruana, D.J.*, Patterning of Metal Oxide thin Films using H2/He Atmospheric Pressure Plasma Jet. Green Chemistry,22, 1406-1413 (2020).Sener, M.E. & Caruana, D.J. Modulation of copper(I) oxide reduction/oxidation in atmospheric pressure plasma jet. Electrochemistry Communications 95, 38-42 (2018).Fowowe, T., Hadzifejzovic, E., Hu, J.P., Foord, J.S. & Caruana, D.J. Plasma Electrochemistry: Development of a Reference Electrode Material for High Temperature Plasma. Advanced Materials 24, 6305-6309 (2012).Calleja, M., Elahi, A. & Caruana, D.J. Gas phase electrochemical analysis of amino acids and their fragments. Communications Chemistry 1, 48 (2018).

Electron Attachment to Cytosine: The Role of Water.

We present an EOM-CCSD-based quantum mechanical/molecular mechanical (QM/MM) study on the electron attachment process to solvated cytosine. The electron attachment in the bulk solvated cytosine occurs through a doorway mechanism, where the initial electron is localized on water. The electron is subsequently transferred to cytosine by the mixing of electronic and nuclear degrees of freedom, which occurs on an ultrafast time scale. The bulk water environment stabilizes the cytosine-bound anion by an extensive hydrogen-bond network and drastically enhances the electron transfer rate from that observed in the gas phase. Microhydration studies cannot reproduce the effect of the bulk water environment on the electron attachment process, and one needs to include a large number of water molecules in the calculation to obtain converged results. The predicted adiabatic electron affinity and electron transfer rate obtained from our QM/MM calculations are consistent with the available experimental results.

Probing Molecular Excited States by Atomic Force Microscopy.

By employing single charge injections with an atomic force microscope, we investigated redox reactions of a molecule on a multilayer insulating film. First, we charged the molecule positively by attaching a single hole. Then we neutralized it by attaching an electron and observed three channels for the neutralization. We rationalize that the three channels correspond to transitions to the neutral ground state, to the lowest energy triplet excited states and to the lowest energy singlet excited states. By single-electron tunneling spectroscopy we measured the energy differences between the transitions obtaining triplet and singlet excited state energies. The experimental values are compared with density functional theory calculations of the excited state energies. Our results show that molecules in excited states can be prepared and that energies of optical gaps can be quantified by controlled single-charge injections. Our work demonstrates the access to, and provides insight into, ubiquitous electron-attachment processes related to excited-state transitions important in electron transfer and molecular optoelectronics phenomena on surfaces.

Theoretical insights into the dissociation process for dissociative electron attachment to adenine and its tautomer

AbstractThe ab initio molecular dynamics (MD) simulations using an atom-centered density matrix propagation method are carried out in the first time to investigate the dissociative electron attachment (DEA) processes of adenine and its tautomer in the gas phase. Since the incoming electron are captured on the lowest π∗ anti-bond orbital, which is led to the different N–H bond, the C–H bond and the C–N bond are broken. The dominant anion observed in DEA dissociation process is the closed-shell dehydrogenated anion (Ade − H)−. The additional anions (Ade − NH2)− and (Ade − 2H)− are also obtained in ADMP simulation. The results are well consistent with the previous DEA experimental results. Thus, the ADMP method is used to gain a more intuitive and better understanding of the necessary dissociation process in the DEA experiment.